Britain's largest trade union claimed it was the victim of a "landmark attack" on the right to strike yesterday after the high court blocked 20 days of walkouts by British Airways cabin crew.

In a verdict that stunned the Unite union, Mr Justice McCombe granted the third injunction against a major transport strike in six months as he overturned an 81% vote in favour of industrial action.

Hours before staff were due to start the first of four five-day walkouts at midnight, the high court ruled that the strike would breach the 1992 Trade Union Act because Unite's attempts to inform members of the poll result were inadequate. The strikes would have affected 1.8 million passengers and cost BA nearly £140m.

A BA spokesman said: "We are delighted for our customers that Unite's plans for extreme and unjustified strike action cannot go ahead." However, the airline said Heathrow customers faced disruption until the weekend because it is too late to unwind limited schedules designed to cope with a prolonged walkout.

The implications of the verdict reverberated around the trade union movement last night, following injunctions against a 12-day Christmas strike by BA cabin crew and a national rail strike over Easter granted on similar technical grounds.

The joint general secretaries of Unite, Tony Woodley and Derek Simpson, said: "This judgment is an absolute disgrace and will rank as a landmark attack on free trade unionism and the right to take industrial action. Its implication is that it is now all but impossible to take legally protected strike action against any employer who wishes to seek an injunction on even the most trivial grounds." Unite said it will apply for an appeal against the injunction in the hope it can reinstate the strikes.

The lawyer credited with the legal strategy that underpinned the defeat of the BA Christmas strike warned that trade unions face considerable difficulty in complying with the 1992 act. "This reinforces the fact that unions have got to get it right and that companies will be increasingly watchful for any errors," said Marc Meryon, industrial relations partner at Bircham Dyson Bell. "It shows that the law is very technical and that the unions have to work hard to comply with it. But that is part of the price they pay for some of the privileges that they have under the act, including the right to call strikes without liability for the damage which they cause."

The BA Christmas strike and the national rail walkout by the RMT union were blocked in the high court because of voting irregularities that are strictly prohibited in the 1992 act. According to legal experts, dozens of industrial disputes have been halted or delayed over the past two years on the same basis.

The judge said he felt "a sense of sadness" that the airline and Unite – which represents more than 90% of BA's cabin staff – had not been able to resolve a dispute over reductions to onboard staffing levels. But he ruled that a Unite strike ballot in February had not complied with a particularly arcane point of law which orders unions to notify not just the result of any ballot to all eligible voters, but also the exact breakdown of votes, including spoiled ballot papers.

He also gave some hope for Unite's efforts at the court of appeal today, stating that the case was finely balanced. McCombe granted an interlocutory injunction on a "balance of convenience", because the cost and impact of not blocking the strike would have been far greater for BA and its passengers, with the airline on course for a two-year loss of £1bn.

Yesterday's ruling could expose Unite to a significant compensation claim because the February ballot was followed by seven days of walkouts in March that cost BA at least £43m. This time round Unite had planned four waves of five-day strikes from 18 to 22 May, then from 24 to 28 May, 30 May to 3 June, and 5 to 9 June in what would have been BA's worst ever bout of industrial unrest.

The injunction was part of a double whammy of good news for passengers yesterday as the Civil Aviation Authority relaxed the safety regime for flights affected by volcanic ash.

Unite's national officer, Steve Turner, said the union was "bitterly disappointed" with the result and would immediately appeal against the decision. "It's an affront to democracy and our members and we will be fighting back," he said. Turner confirmed that, as the situation stood, the strikes by 11,000 cabin crew would be called off. The transport secretary, Philip Hammond, said: "This decision is good news for passengers. However, more strikes are planned and I have met with British Airways and Unite today and made it clear that both parties must get back round the negotiating table urgently."

BA's chief executive, Willie Walsh, and the Unite joint general secretaries held talks at the Acas conciliation service yesterday, which broke up without agreement shortly after the high court ruling.

Emerging from the Acas talks, Walsh said the injunction gave Unite an opportunity to "pause and reflect". He added that he hoped to run a 100% service if Unite calls another wave of strikes in a dispute now mired in a row over the withdrawal of staff travel perks from the 5,000 cabin crew who took part in the March strikes. "What I want to do is to fly 100% of the schedule and to remove any uncertainty. But equally it is important that we do that while at the same securing the long-term viability of BA, and that can only be achieved if we progress on permanent structural change to our cost base."

The TUC's Brendan Barber said: "This is a desperately worrying judgment. A strike that clearly has majority support has been turned over on a tiny technicality. This, and other recent decisions, begin to make it look as if there is no effective right to strike in today's Britain."

Bob Crow, general secretary of the Rail, Maritime and Transport union, which also called off industrial action after a legal challenge this year, said: "Today's decision effectively outlaws strike action. It's another turn of the screw of the anti-union laws."